1. Field of the Invention
This invention relates to a process of recovering pure benzene from mixed hydrocarbons containing the same and gaseous and difficultly condensible nonaromatic compounds, by extractive distillation with a selective solvent.
2. Discussion of Prior Art
For a recovery of pure aromatic compounds from mixed hydrocarbons by extractive distillation with a selective solvent it is known to feed the feedstock to be separated to a stabilizing column, which precedes the distillation column, to withdraw a high-boiling residue from the sump of the stabilizing column and to withdraw mixed vapors of aromatic and nonaromatic compounds from the stabilizing column at its lower part or some plates above its inlet for the feedstock to be separated. To control the evaporation conditions, the mixture is then fed in the form of a vapor or in an entirely or partly liquefied form to an extractive distillation column at one or more points and the pure aromatic compounds together with the selective solvents are withdrawn from the sump of the extractive distillation column and subsequently separated (see German Offenlegunsschrift No. 2,263,344).
In another known process of recovering pure aromatic hydrocarbons by extractive distillation with a selective solvent, the benzene and toluene fractions are simultaneously withdrawn as overhead vapors from a pretreating column and as a vapor are supplied to an extractive distillation column.
The nonaromatic compounds are withdrawn as an overhead product and the extraction solvent together with benzene and toluene is withdrawn from the sump of the extractive distillation column, which is succeeded by a stripping column. The extracting solvent is thus separated from benzene and toluene in the stripping column and becomes as a sump product. The benzene and toluene, which become available as overhead products, from the stripping column, may subsequently be separated in known manner by distillation, if desired (German Offenlegungsschrift No. 2,313,773).
When the nonaromatic compounds have an initial boiling point of about 30.degree. C., extractive distillation with the aid of the solvent may be used for an exact separation into nonaromatic compounds and pure benzene. But owing to the low boiling point of the low-boiling nonaromatic component, the extractive distillation column must be operated under superatmospheric pressure so that all temperatures are increased and a heat exchange between hot solvent from the sump of the stripping column and the sump product of the extractive distillation column can no longer be effected.
In such cases the feedstock is prepurified with the solvent in a preceding extractive distillation column, which is operated under such superatmospheric pressure that its overhead product consisting of the low-boiling components of the feedstock condenses at a temperature which permits an economical condensation of the overhead product. That extractive distillation is so controlled that no benzene is lost in the overhead product. It is not endeavored to obtain by said extractive distillation a sump product consisting of solvent and benzene. On the contrary, all nonaromatic compounds having an initial boiling point at 60.degree. or 65.degree. C. are also collected in the sump.
The feedstock is then separated into nonaromatic compounds or raffinate and pure benzene (extract) in a succeeding second extractive distillation effected by means of the solvent. That second extractive distillation is effected under a slightly subatmospheric pressure so that the heat of the hot solvent from the sump of the stripping column can be fully utilized for heating the second extractive distillation column and also for heating the first extractive distillation column because the latter may be supplied with solvent at a very low rate so that its sump temperature is so low that an effective heat exchange is possible.
That process cannot be used for an economical recovery of pure benzene if the feedstock consists of components which can be condensed only with very great difficulty, i.e., when part of the feedstock is gaseous under atmospheric pressure.
Those components which remain gaseous or are difficultly condensible can be condensed only under very high pressure or with the aid of a refrigerant. A sufficiently high pressure involves a great increase of the distillation temperatures so that high-pressure steam or tubular heaters must be used to generate the heat required for distillation. In that case the removal of the low-boiling hydrocarbons is controlled by a pressure-regulating valve so that the gaseous state is resumed in most cases after the pressure regulation. This may involve a loss of benzene to be recovered.